With increasing interest in greener alternative processes to produce chemicals and materials, cyclic carbonates, which are potential monomers for the environmentally benign production of aliphatic polycarbonates and polyurethanes by ring-opening polymerization (ROP), have attracted attention in recent years. These aliphatic polymers, besides having traditional applications in engineering, optical devices, seatings, seals, coatings and high performance adhesives, are expected also to find use in biomedical field because of their biocompatibility and low toxicity.
Aliphatic polycarbonates are currently produced industrially from an alkanediol, such as 1,6-hexanediol, and phosgene, triphosgene and dialkyl carbonates. Polyurethanes are produced using polyols, such as alkanediols and glycerol, and isocyanate, which is derived from the reaction between an amine and phosgene. Since phosgene and low-molecular weight isocyanates have undesirable toxicological profiles, attempts have been made to develop routes to make polyurethanes from other sources however none of these have yet been commercially established. A demand has now emerged for isocyanate free polyurethanes for different applications using environmentally friendly production process.
Lately, a number of reports have appeared on the synthesis of cyclic carbonates by a phosgene-free route, but have focused on five-membered cyclic carbonate including addition of carbon dioxide to epoxides using metal containing catalysts under pressure, and by transesterification of polyols with dialkyl carbonate using metal-, or enzymatic (lipase) catalysis. For use in ROP process, however, six-membered cyclic carbonates are preferred to the five-membered one because of being less thermodynamically stable than its ring-opened polymer and thus retaining CO2, during the polymerization process.
Synthesis of six-membered trimethylene carbonate is traditionally achieved by reacting 1,3-propanediol with phosgene or its derivatives. Among the other reactions studied, metal catalysed coupling of oxetane such as trimethylene oxide with carbon dioxide has given high yields of trimethylene carbonate. A method of cyclic carbonate synthesis from 1,3-propanediol and ethyl chloroformate in the presence of a stoichiometric amount of triethylamine has been reported. Transesterification of 1,3-propanediol with dialkyl carbonate catalyzed by metal or organo-catalysts has been proposed as a more environmentally benign procedure. Syntheses of six-membered cyclic carbonates with functional groups from poly-functional alcohol such as trimethylolpropane (TMP) or pentaerythritol (PE) have required more complicated methods with low yields. Polycyclic six-membered carbonates could be prepared by radical polymerization of acrylic monomers with pendant cyclic carbonate groups. As a different approach, tris- or tetrakis (alkoxycarbonyloxy) derivatives, obtained from catalytic transesterification of trimethylolpropane (TMP) and diethyl carbonate (DEC), have been subjected to thermal disproportionation using Aerosil 200 at 200-220° C. followed by distillative depolymerisation under reduced pressure, to give the cyclic product 5-ethyl-5-ethoxycarbonyloxymethyl-1,3-dioxan-2-one with a low yield.
Lipase catalysed synthesis of the six-membered cyclic trimethylene carbonate from 1,3-diol and dimethyl or diethyl carbonate has been achieved in a solvent system of acetonitrile and toluene (4:1, v/v) using very high concentration (600-900% w/w of the diol) of the immobilized Candida antarctica lipase B, Novozym®435 (N435), however with moderate yield (53%) and low productivity.
Synthesis of cyclic carbonates with functional groups using lipase-mediated reaction between TMP and dialkyl carbonate in solvent free condition has recently been reported. The product formed was a mixture of cyclic- and linear (mono-, di- and tri-) carbonates, the proportions of which depended on the reaction conditions used. Subjecting the product mixture to thermal cyclization at 70-90° C. without the biocatalyst converted the linear carbonates to the cyclic ones. However these enzymatic reactions have limitation of high enzyme cost.